Microwave apparatus for circular polarization



C. DRISCOLL April 10, 1956 MICROWAVE APPARATUS FOR CIRCULAR POLARIZATIONFiled May 8 1951 FIG? FIG.5

FIG.6

INVENTOR.

CLARE DRISCOLL FIG.I

FIG. 3

United States Patent MICROWAVE APPARATUS FOR CIRCULAR POLARIZATION ClareDriscoll, Washington, D. C., assignor m the United States of America asrepresented by the Secretary of the Army Application Ma a, 1951, SerialNo. 225,211 4 Claims. c1. 333-21 This invention relates in general tomicrowave waveguides and is particularly directed to a broad bandtransformer for converting plane polarized waves to waves of circularpolarization.

An electrical wave is said to be circularly polarized when the fieldthereof can be resolved into two equal components 90 apart in space and90 apart in time. If the two vectors are not of equal amplitude, or ifthe time difference between them is other than 90, the wave is said tobe elliptically polarized; in the special case in which the time phaseangle reduces to zero, the wave is linearly polarized. Thus, both linearpolarization and circular polarization can be looked upon as specialcases of elliptical polarization.

The equation of this ellipse may be expressed as follows:

tude, and 6 is the time phase angle of the two components.

=sin 5 and A=B so that the components are 90 apart in time phase and areof equal amplitude the equation of the ellipse reduces to E: +Ey =Awhich is the equation of a circle. The wave is then circularlypolarized. If 6:0 or 1r (components in phase or 180 out of phase), theequation becomes that of a straight line the wave is linearly polarized.

Various methods may be used to produce a field having components ofsubstantially equal amplitude 90 apart in space in a square orrectangular wave guide. For example the two components may be separatelyintroduced into the waveguide or they may be generated by applying alinearly polarized wave to the waveguide having an electric vectorrotated approximately 45 with respect to either of the two pairs ofsides of the square or substantially square waveguide, so that theelectric vector will have substantially equal components perpendicularto each pair of sides.

It is necessary, in order to produce a circularly polarized wave, tocreate a phase shift in time of 90 in the two components of the wave inspace quadrature. This has been previously accomplished by utilizing arectangular waveguide of asymmetric proportions or, in a squarewaveguide, by the introduction of a longitudinal dielectric slabinserted parallel to one pair of sides of the waveguide.

The dimensions of the waveguide or of the dielectric slab are chosen soas to produce the desired time phase shift at the center of the desiredfrequency band. At frequencies above and below the center frequency,however, the time phase-shift will vary from the desired 90 timephase-shift, causing the polarization to deviate from circularpolarization at these frequencies. The frequency 2,741,744 Patented Apr.10, 1956 'ice band in devices of this nature is, therefore, restrictedby the limits of ellipticity which are considered satisfactory.

It is consequently an object of this invention to provide apparatuswhich overcomes the limits of the prior art by reducing the deviationfrom circular polarization over a given frequency range.

It is a specific object of this invention to provide improved apparatusfor producing circular polarization within desired limits of ellipticityover a broad frequency range.

.In accordance with this invention two wave guide transformers areconnected in series to produce a total time phase shift between thetransformers which is subject to less variation with change in frequencythan would occur through the use of a single waveguide transformer. Onetransformer is designed to produce a time phase shift which is greaterthan the desired phase shift. The other transformer is designed toproduce a correcting phase shift which reduces the total phase shift toapproximately the desired value over a broad frequency range.

The foregoing and other objects of the invention will be best understoodfrom the following description of specific embodiments thereof,reference being had to the accompanying drawings, wherein:

Fig. l is an isometric view of one embodiment of the invention;

Fig. 2 is an isometric view of another embodiment of the invention;

Figs. 3, 4 and 5 are cross sectional views of the arrangement of Fig. 1;and

Figs. 6 and 7 are cross sectional views of the arrangement of Fig. 2.

- Referring now to Fig. 1, a view of a device for converting a linearlypolarized wave propagated in space quadrature into a substantiallycircularly polarized wave is shown, said device comprising a rectangularwaveguide section 11 of constant cross section, as shown in Fig. 3,having a pair of long sides and a pair of short sides. A second section13 of rectangular wave guide of constant cross section, as shown in Fig.5, is provided having a length and a dimension ratio of long side toshort side which difier from the length and dimension ratio of sides ofsection 11. The dimensions of the narrow walls of the sections 11 and 13are, of course, large enough to support propagation in which theelectric field voltage vectors are perpendicular to the narrow walls forotherwise a circularly polarized wave would not be produced by thewaveguide.

The long sides of section 11 are joined to the short sides of section 13and the short sides of section 11 are joined to the long sides ofsection 13 by tapered waveguide section 15. Waveguide section 15 changesin cross sectional dimension along its length from the dimensions ofsection 11 shown in Fig. 3 through a square cross section shown in Fig.4 to the dimensions of section 13 shown in Fig. 5.

A wave propagated into the open end of waveguide section 11 in spacequadrature may be derived from any desired source (not shown) in aconventional manner. This wave has a composite electric vector lying inthe cross sectional plane of the waveguide and inclined at an angle ofapproximately 45 to both pairs of sides of the rectangular waveguide.This electric vector can be resolved into two components Ex and B eachperpendicular to a separate pair of sides of the waveguide.

At the open end of waveguide section 11 Ex and By are still in timephase and the resultant is a linearlypolarized wave. In order to satisfythe condition for a circularly polarized wave the two components mustalso have a time phase of or 270.

The waveguide section 11 as seen by the component B:

is different in width from that seen by B Therefore their velocities ofpropagation are somewhat different, and in thetpassage of the componentsthrough section 11 the phase angle between -Ex and B progressivelydeparts from zero' and the wave becomes elliptically polarized.

This progressive increase in time phase angle continues with the passageof the wave into the tapered waveguide section 15 up to the 'point insection 15 where the 'dimem sions of the two pairs of sides become'equal. At this point, indicated by the cross sectional View of Fig. -4,the components Ba and By see equal dimensions, and past thispoint insection 15 and in waveguide "section--13, it is apparent that componentsEx and By see a reversal in pro Frequency: Phase shift, degrees To 9fo-l-Af 75 fo-Af 110 In order to narrow the time phase shift over thedesired frequency range the length of this Waveguide section may bedoubled so as to produce a time phaseshift of 180 at its centerfrequency. This wave guide 'will have the following characteristics:

Frequency: Phase "shift, degrees f0 180 ftH-Af 150 f0-A), 220

connected waveguide section corresponding to section 13 of Fig. 1 havingdifferent cross sectional dimensions and therefore a difierent ratio ofphase shift to frequency variation. Tfiiis section can be designed tohave the following characteristics:

Frequency: Phase shift, degrees f minus 90 tori-n? minus 55 fuhjf minus125 The total phase shift of these 'two sections connected in serieswould be:

Frequenc f0 r w :2 "yo-A7 95 r This is a substantial improvement overthe phase shift variation over the-same frequency range obtainable froma single waveguide section.

lt-should be noted that'in theapar'ticular construction disclosed inFig. 1 the tapered section willalso produce a-shift in the-time phaseangle. 7 This must be taken into consideration in the design of'seetions1'1 and 13. This phase shift will be first-in one direction and then inthe other, reversing at the plane in which the cross section ofthe'taperis symmetn'ca1. Section 15 canbe-cdnsidered as two zguide sections, eachsectionof whichcanbe considered'as'havinga taper to asymmetrical endwhich-is mated with the facing end-of the other sections. Carrying thispoint of view -to:its limit, :none of thewa'veguide sections 'wouldbeuniform throughoutdts complete length and the entire ipola'rizationprocess could' becarited out in a properly dimensioned taper with nostraight se'ction. Fig. 'Z' ShOWS a viewsot a second :embodime'nt'of"the Phase shift, "degrees ing the dimensions and thedielectric-constant of the slabs th'ewtivghide. I I

by placing a second longitudinal dielectric slab having differentdimensions in a succeeding section 21 of the waveguide parallel to'thesec'ondpairofsid'es. 13y selecta variation of phase shift overthetdesired frequency range may 'be obtained which is similar to thatpreviously described for the "structure illustrated in Fig. 1. Whilethere have been described "what are presently considered to be preferredembodiments of this invention, it will be obvious to those skilled inthe art that various changes and modifications may be made thereinwithout departing from the invention. a

What is claimed is: -1. A device :for introducing in a band offrequencies asub'st'a'ntial'ly 90 "time phase'displacement'betweenthespace quadrature com onents of the plane-polarized Waves'of said band of-frequencies comprising first hollow waveguide means for increasing therelative time phase angle between the space quadrature components of thecenter frequency of said band of frequencies by a given amount greaterthan 90 and shifting the relative'time 'phase angle between the spacequadrature components quency variation which differs from that of saidfirst waveguide means for reducing the shift in relative time phaseangle between the space quadrature components of the output waves ofsaid first waveguide means or all frequencies in'said band tosubstantially 90.

2. A device as claimed in claim 1 wherein said 'first means comprises awaveguide section having asymmetrical dimensionsand'said second meanscomprises a waveguide section of asymmetrical but dilferent dimensionsthan said first 'waveguidemeans. l

3. A device as cla'imed in claim 1 whereinsaid'first means comprises awaveguide rectangular in cross secti'o'n andhavingrretativelygreaterspacing between its side walls than :"s'pacing:b'etweeniits top and bottom surfaces, said ise'c'ond me'an's comprises"a waveguide rectangular in cross section V and having relativelygreater 'spacing betw'een i'ts top and bo'ttom :surltaces' than thespacing between its si'd'e walls, and :further including a waveguidecomponent formed to join the opp'os'i'ng 'short "walls-of eachrectangularwaveguidesection with'the opposing long walls of the otherrectangular waveguide section.

4. A device as claimed in claim 1 wherein said first andsecon'dwaveguides are '-'square in 'cr'o's's section and further including :afirst dielectric :s'lab mounted within said fir st waveguideparall'ellto the 'walls "thereof anda

